Conventional passive air samplers (PAS) and passive dry deposition
samplers (PAS-DD) were deployed along a 90 km south–north transect at five sites in
the Athabasca oil sands region (AOSR) during October to November 2015. The purpose
was to compare and characterize the performance of the two passive sampling
methods for targeted compounds across a range of site types. Samples were
analyzed for polycyclic aromatic compounds (PACs), nitrated polycyclic
aromatic hydrocarbons (NPAHs), and oxygenated PAHs (OPAHs).
Application of passive air sampling techniques has become widespread due to
their simplicity, convenience, and cost-effectiveness. It enables us to
routinely monitor air pollutants at a larger scale and to extend air
monitoring networks to strategic sites that are not feasible for active air
sampler installation. Illustrated in Fig. S1 in the Supplement, conventional double-domed
polyurethane foam (PUF) disk passive air samplers (PAS) have been commonly
used in several air monitoring programs, including the Athabasca oil sands
air monitoring network, which uses the Global Atmospheric Passive Sampling
(GAPS) Network-type sampler (Harner et al., 2006; Pozo et al., 2004;
Klánová et al., 2006; Jaward et al., 2004;
Schuster et al., 2015). A recent
study demonstrated that the GAPS-type polyurethane foam (PUF)-PAS
was capable of accumulating particles, ranging in size from 250 to 4140 nm,
with no discrimination compared to conventional PS-1-type active air
samplers (Markovic et al., 2015). The geometry of the PUF-PAS allows it to
capture fine particles (aerodynamic diameter
Starting in October 2015, PAS-DD were co-deployed with PAS at five sampling sites in the Athabasca oil sands region (AOSR) in order to compare the performance of the two samplers. These sampling sites are part of a larger 16-site passive air monitoring network that has been operating since 2010 (Schuster et al., 2015) under the Canada–Alberta Oil Sands Monitoring (OSM) plan and reporting on polycyclic aromatic compounds (PACs) (i.e., parent and alkylated PAHs; dibenzothiophene, DBT; alkylated DBTs; retene, RET); PAH derivatives, including nitrated polycyclic aromatic hydrocarbons (NPAHs) and oxygenated PAHs (OPAHs); and an assessment of the toxicity potential of the chemical mixture (Schuster et al., 2015; Jariyasopit et al., 2016).
As a result of an increase in oil sands production, there has been growing
concern over impacts of organic constituents in air, their transport and
deposition, and the associated impact on the health of the environment and
on humans. One of the important classes of organic pollutants in this
context is the PACs. PACs are emitted from a variety of sources such as
combustion processes (e.g., forest fires, trash burning) and also petrogenic
sources; they are present in the bitumen-containing ore that is mined in the
AOSR (Yang et al., 2011). Information on PAC sources can be obtained from
the National Pollutant Release Inventory (NPRI) and environmental impact
assessment (EIA), but these are limited to PACs which do not account for
compounds produced by transformation reactions (NPRI,
Petcoke is a solid residue and is a byproduct of the upgrading of bitumen after lighter hydrocarbon molecules have been fractionated. There are two major types of coking in the AOSR referred to as “delayed” and “fluid” coking processes (Anthony, 1995). Both coking processes involve thermal cracking of the feedstock to extract lighter products and leave behind petcoke. In the delayed coking process, the cracking process continues, after a short thermal cracking in a furnace, in coke drums where solid coke is produced. In the fluid coking process, the coke produced in a heated reactor is circulated between the reactor and a burner to transfer heat. The delayed coking process occurs at lower temperature than the fluid coking process; therefore, the delayed petcoke contains more volatiles and potentially more PACs than the fluid petcoke.
The majority of petcoke produced in the AOSR has been stockpiled while only a small percentage is reused on-site as fuel (Alberta Energy Regulator, 2018). Recently petcoke has been used for capping decommissioned tailings ponds, which greatly enhances its surface area available for erosion and evaporation (Alberta Energy Regulator, 2018). We hypothesize that secondary emissions to air of PACs from oil sands ore (open-pit mines) and/or petcoke stockpiles, either through evaporation or particle suspension in air (e.g., wind transport), contribute substantially to PAC burdens in air, especially in nearby source areas. Therefore, in addition to comparing the performance of PUF-PAS and PAS-DD samplers, a secondary objective of this study is to assess the extent to which oil sands ore and petcoke contribute to the PAC burden of air in the AOSR.
Passive sampling site map (110 km
Five sampling sites (Fig. 1) are part of the passive air monitoring network
in the AOSR. Details regarding site locations, sampling media preparation,
and sample deployment have been previously described (Schuster et al.,
2015). The samplers were mounted approximately 3 m above the ground. In
brief, PUF disks were pre-cleaned with accelerated solvent extraction
(Dionex ASE 350) using acetone, petroleum ether, and acetonitrile, prior to
use. Since October 2015, PAS-DD have been deployed alongside
PAS at a subset of five sites (Figs. 1 and S1). PAS-DD was previously demonstrated to collect gas-phase PAHs (i.e., dry
gas-phase deposition) at similar rates as PAS (Eng et al.,
2013). A sampling rate of about 5 m
The monitored PACs, NPAHs, and OPAHs are listed in Table S1 in the Supplement. Standards for
the target analytes were purchased from Cambridge Isotope Labs (Andover,
MA), Chiron (Trondheim, Norway), and AccuStandard (New Haven, CT).
Deuterium-labeled recovery and internal standards were purchased from
Cambridge Isotope Labs (Andover, MA) and CDN Isotopes (Point-Claire, Québec,
Canada). The deuterated recovery surrogates included
2,6-dimethylnaphthalene-d
PUF disk, fluid petcoke (
Analysis using scanning electron microscopy coupled with energy dispersive X-ray
spectroscopy (SEM-EDS; Zeiss Sigma 300 VP-FESEM) was carried out at the
University of Alberta Earth and Atmospheric Sciences SEM lab. A pie-shaped
wedge section of the PUF disk (2 cm base) was used for the SEM-EDS analysis.
Particles entrained in the PUF wedges were removed by ultrasonication in
dichloromethane, which was subsequently dried by nitrogen gas. A portion of
the dried particles was transferred to double-sided adhesive conductive tape
for SEM-EDS analysis. The EDS spectra were acquired by a Bruker energy EDS
system with dual silicon drift detectors, each with an area of 60 mm
Results were corrected to account for the wedge portion removed for the SEM
analysis and also for the area of the PUF disk covered by the open plate and
perforated support that holds the PUF in place (representing about
The highest
concentrations in air for
Composition of target compounds in air samples from five sites in the oil sands region, collected using the conventional passive sampler (PAS) and the passive dry deposition sampler (PAS-DD), collected during October to November 2015. “PAC” includes parent PAHs and alk-PAHs.
For all the sites,
NPAHs have been previously measured in PM
For OPAHs, the highest concentration was found at AMS6 which is located in the
town of Fort McMurray, approximately 30 km south of the main mining area.
This is consistent with our previous study. The elevated OPAHs at this site
are due to local primary combustion sources (e.g., vehicular exhausts) as
well as the enhanced atmospheric transformation process which is dependent on
gaseous oxidants emitted from the combustion sources (Jariyasopit et al.,
2016). The highest concentrations for
A key aspect of the study was to compare
the performance of PAS and PAS-DD to capture PACs, NPAHs, and
OPAHs. Their relative performance can be illustrated using the enhancement
ratio which is defined as the ratio of the concentration of an analyte in
PAS-DD to that in PAS. A value close to 1 indicates
comparable ability of the two sampler types for capturing an analyte. Figure 3
shows enhancement ratios for PACs for all the sampling sites. Average
ratios for 2–3-ring, 4-ring, and 5–6-ring PACs were 2.3, 2.8, and 3.6
respectively. Similarly, the enhancement ratio increased with molecular
weight for the NPAHs (Fig. S3) up to an average value of about 4. Comparison
of the enhancement ratios of PACs among the sites indicates that the ratio is
partly dependent on particle loadings. This is evident from higher
enhancement ratios of higher-ring PACs, except for BghiP (Benzo(ghi)perylene), observed in AMS5, which is the near-source site where
Enhancement ratios (expressed as a ratio of the concentration of
an analyte in PAS-DD to that in PAS) for
The enhancement ratios for the NPAHs and OPAHs were considerably more variable among sites compared to the patterns observed for the PACs (Fig. S3). This may be due to multiple factors contributing to their presence in air, which can be by direct emission from primary sources as well as production in air through transformation processes. Transformation reaction rates will vary among NPAH and OPAH compounds and also spatially, depending on atmospheric conditions and oxidant concentrations.
The composition of PACs, NPAHs, and OPAHs in petcoke and oil sands ore samples was investigated in order to assess if these compositions are reflected in the passive air samplers, thereby indicating potential contributions. Results of residue analysis are discussed below and summarized in Table 1. Individual PAC concentrations and composition are given in Table S3 and Fig. S4.
Concentrations of parent PAHs, dibenzothiophene (DBT), alk-PAHs,
retene (RET), alk-DBTs, NPAHs, OPAHs, and potential NPAH markers in fluid petcoke,
delayed petcoke, oil sands ore (ng g
N.D.: non-detect.
The levels of
PAC compositions of fluid petcoke, delayed petcoke, oil sands ore,
and air samples (PAS and PAS-DD). PAC compositions of air samples are
averages of five sites. The number at the top of each bar represents the
total residue concentration (ng mg
For the petcoke and oil sands ore samples, residues of the sum of NPAH and OPAH concentrations
were 2 to 4 orders of magnitude lower than the PAC concentrations. Similar to the results for PACs in petcoke samples, the oil
sands ore exhibited higher
GC-MS extracted ion chromatograms of 4-nitrobiphenyl (4-NBP), 2-nitropyrene (2-NP), 1,6-dinitropyrene (1,6-NP), and 6-nitrobenzo(a)pyrene (6-NBaP) in a selection of samples including delayed petcoke, fluid petcoke, oil sands ore, passive dry deposition sample at AMS5, passive dry deposition sample at AMS14, and air sample collected from the ore–air partitioning study. The phase distribution of each marker is also indicated as gas phase or particle phase.
The delayed petcoke exhibited higher
residues of
Comparisons of PAC compositions for the delayed petcoke, fluid petcoke, and
oil sands ore versus the PUF-PAS, and PAS-DD air samples revealed interesting
differences as shown in Figs. 4 and S4. For instance, the petcoke particles
exhibited enrichment in the higher-molecular-weight, semi-volatile, and
particulate-associated parent PAHs and alk-PAHs, while the oil sands ore and
passive air samples were dominated by lower-molecular-weight and
more-volatile alk-PAHs. DBT makes up a small proportion (
To assess the potential contributions of particle-associated PACs in air,
stemming potentially from either petcoke or the exposed ore from open-pit
mines, we focus on the higher-molecular-weight compounds which exist
primarily in the particle phase. These compounds dominate the PAC composition
of petcoke (Fig. S4), since the more-volatile, lower-molecular-weight
compounds are depleted during the high temperature coking process. If petcoke
and/or ore particles represent an important contributor to PACs present in
air, then their compositions should be reflected in PAS and especially the
PAS-DD samples. The compositions of the 4–6-ring PACs in delayed petcoke,
fluid petcoke, oil sands ore, and passive air samples are compared in
Fig. S5. The parent 4–6-ring PAH composition of the passive air samples did
not match the 4–6-ring PAH compositions of the petcoke and ore, suggesting a
minimal contribution of these sources to parent PAH burdens in air. This
implies that other sources of parent PAHs (e.g., combustion, vehicle
emissions) were more dominant, whereas the 4–6-ring alk-PAH compositions for
all samples were more similar, suggesting some contribution of petcoke and
oil sands ore particles in the passive air samples. However, these findings
for parent PAHs and alk-PAHs are somewhat contradictory. If petcoke particles
had contributed substantially to alk-PAHs in air, then the parent PAHs
contained in these same particles (and making up
In the case of NPAHs and OPAHs, their relatively low residue concentrations in petcoke and ore samples complicate the assessment of potential contributions of these particles to air samples. As shown in Fig. 5, the high molecular weight and particle-associated NPAH marker compounds, 2-NP, 1,6-DNP, and 6-NBaP are detected in various petcoke and ore samples but not reflected in air. However, the gas-phase marker compound 4-NBP which is present in the delayed petcoke and the oil sands ore, is also captured at site AMS5 and AMS9 (Table S2). These two sites are the closest to open-pit mines, which points to the potential importance of volatilization from open-pit mines as a source to air of 4-NBP and other volatile PACs. This finding is supported by the results of a simple ore–air partitioning experiment showing that 4-NBP was in fact detected in air that has equilibrated with ore. Details of the experimental setups which are based on Francisco et al. (2017) are provided in the Supplement.
We conclude that oil sands ore is contributing substantially to burdens of PACs in air near mining areas but not at sites further removed from open mines. This finding is consistent with air and snow monitoring studies that indicate that most of the deposition of mining-related particles and associated chemicals occurs within the first several kilometers of mining areas (Schuster et al., 2015; Kelly et al., 2009).
Scanning electron microscopy (SEM) images and energy
dispersive X-ray (EDS) spectra of
In this aspect of the study,
passive air samples were screened for petcoke particles using scanning
electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS).
Petcoke particles can be distinguished from ore and other particle types by
their unique morphology (using SEM) and elemental composition (based on EDS
spectra). An image and EDS spectrum of an authentic delayed petcoke particle
is shown in Fig. 6a, demonstrating the unique relative elemental abundance
(excluding carbon) as S (sulfur)
In summary, this study demonstrated the performance of PAS-DD in capturing depositing particles that are enriched with the higher-molecular-weight PACs and PAC derivatives. Due to the design of PAS-DD where the PUF disk is shielded from precipitation and direct sunlight by a cover plate, PACs captured by PAS-DD reflect dry deposition of PACs that have been protected from photolytic degradation. Alternatively, environmental passive samplers such as peat and moss have been used to collect wet and dry depositions (Zhang et al., 2016). These environmental passive samplers are exposed to direct sunlight but potentially collect more of the dry deposited fraction, especially the very large particles which may be excluded by PAS-DD due to the top cover plate. The environmental samplers also accumulate PACs deposited in precipitation. Therefore, we consider PAS-DD, peat, and moss as complementary tools for assessing ecosystem impacts through atmospheric deposition. The comparisons of chemical composition of PACs in passive air samples with petcoke and oil sands ore samples demonstrated an important contribution of oil sands ore to PAC concentrations in air for sites that were closer to open-pit mining areas. Further characterization of ore–air partitioning is identified as a topic for future experimental work and modeling. Lastly, in this study we identify 4-NBP (4-nitrobiphenyl) as a potential marker chemical of oil sands ore and delayed petcoke.
Data used in this study are provided in the Supplement.
The supplement related to this article is available online at:
NJ was responsible for the study design and conducting PUF-PAS and PAS-DD sample preparation, sample analysis, data analysis and preparation of the manuscript; YZ and JM contributed to the analysis and interpretation of the petcoke samples and XRF analysis; TH was the study lead and contributed to the study design and data interpretation and reporting.
The authors declare that they have no conflict of interest.
This article is part of the special issue “Atmospheric emissions from oil sands development and their transport, transformation and deposition (ACP/AMT inter-journal SI)”. It is not associated with a conference.
This project was jointly supported by the Climate Change and Air Quality Program of Environment and Climate Change Canada and the Joint Oil Sands Monitoring program. The Wood Buffalo Environmental Association (WBEA) is acknowledged for their support in passive air sample collection. Yifeng Zhang acknowledges financial support from Alberta Innovates. We thank thank Eftade Gaga, Jasmin Schuster, and Elisabeth Galarneau for their comments on the manuscript.Edited by: Jennifer G. Murphy Reviewed by: three anonymous referees